1. Field of the Invention
The present invention relates to a bearing for axial linear motion for allowing relative linear movement between a shaft and an outer cylinder surrounding the shaft, through the intermediary of a multiplicity of balls which minimize the resistance to the movement by virtue of the rolling friction, so as to enable the linear movement with reduced force. The term "bearing" in this specification is used to cover both ball spline bearings which permit reciprocatory relative axial movement while enabling transmission of torque and linear motion bearings which permit relative axial movement but is not designed any torque transmission.
2. Description of the Prior Art
A typical conventional bearing for axial linear motion has an outer cylinder provided on the inner peripheral surface thereof with a plurality of loaded ball guide grooves and non-loaded ball guide grooves each having a substantially U-shaped cross section, the two types of grooves appearing alternatingly on the circumferential direction. The outer cylinder is further provided on the inner peripheral surface thereof with circumferential grooves disposed at both axial ends of the guide grooves, in such a manner that each circumferential grooves interconnect each loaded-ball guide grooves with the associated non-loaded ball guide grooves.
The outer cylinder receives a ball retainer which has a plurality of endless ball grooves each of which corresponds to each pair of loaded and non-loaded ball guide grooves, so that a multiplicity of balls which are held between the outer cylinder and the retainer are circulated through each endless ball passage which is constituted by cooperation between the endless ball groove in the ball retainer and the loaded and non-loaded ball guide grooves in the outer cylinder connected through the circular ball grooves. The ball retainer is rigidly fixed to the outer cylinder by means of stop rings which are fitted in both axial ends of the outer cylinder.
As to further detail of the bearing of this type, a reference shall be made to Japanese Patent Publication Nos. 22210/1978 and 33702/1978.
This known bearing has a drawback in that, since the outer cylinder has to have not only the loaded and non-loaded ball guide grooves, but also the circumferential grooves and the engaging grooves for retaining the stop rings, the overall length of the outer cylinder is increased with a result that the weight and cost of the bearing are increased. Such a heavy bearing is not suitable for use in various machines and instruments.
In addition, it is generally difficult and time consuming to form on the inner peripheral surface of the outer cylinder both the axially extending loaded and non-loaded ball guide grooves and circumferential grooves which are substantially perpendicular to the ball guide grooves. This causes an impediment to the mass production of the bearing of the type described.
Another problem is that the stop rings are indispensable for fixing the retainer, to the outer cylinder. This in turn requires provision of suitable dust seals such as of a rubber, in order to prevent dusts and other foreign matters from coming into the bearing through a clearance around the stop ring. The necessity for the stop rings and dust seals increases the number of steps in the assembly process.
In order to obviate these drawbacks and problems of the prior art, another bearing has been proposed in which the axial length of the outer cylinder is reduced by omitting the circumferential grooves. In this case, a pair of end covers are attached to both axial ends of the outer cylinder. The end covers are provided with ball turning grooves each cooperate with corresponding portion of the associated endless ball groove in the retainer so as to permit the balls to move from the non-loaded ball grooves into the loaded ball grooves and vice versa.
This type of bearing is disclosed, for example, in Japanese Patent Laid-Open Nos. 155922/1980, 159320/1980 and 159321/1980, as well as in Japanese Patent Publication No. 50969/1982.
The end cover of this type of bearing is formed from an alloy by diecasting, and is rigidly secured to the outer cylinder by means of fixing members such as small screws. The retainer also is made of a metallic material having outer configuration and size substantially conforming with the inner configuration and size of the outer cylinder. The fabrication of the outer cylinder and also of the retainer is difficult and the production cost inevitably rises due to such a difficulty.
In order to obviate this problem, it has been proposed to use the retainer and the end covers as plastic materials, and to integrally weld the end covers to both axial ends of the retainer, as disclosed in Japanese Patent Laid-Open Nos. 76547/1977, 54638/1978 and 115622/1980.
In this proposed bearing, however, the end cover has a flat inner peripheral surface and, therefore, the end cover does not have any function for positively guiding the balls along a constant track when the balls are being turned. In consequence, during high speed movement of the bearing, the balls tend to be urged radially outwardly in such a manner as to float up from the retainer, and the balls become free both from the retainer and the end cover. In other words, the balls are not restrained neither in the axial direction nor radial direction when being turned at each axial end of the ball passage, so that they cannot move along a constant orbital path at each axial end of the endless passage. This undesirably impedes the smooth turning of the balls and causes high level of noise due to mechanical interference between the balls.